A conventional spark plug is supplied to a combustion chamber of an internal-combustion engine for the purpose of igniting an air-fuel mixture. The spark plug discharges a spark in a discharge gap formed between a center electrode and a ground electrode that faces a front end of the center electrode.
Such a spark plug is normally fixed to an internal-combustion engine with a male thread of a metal shell of the spark plug engaging with a female thread provided in a plug hole of the internal-combustion engine.
Further, it has been disclosed that a spark plug has a separate male thread formed on an outer face of a cylindrical pressing member, instead of forming it on the metal shell. The male thread of the cylindrical pressing member is engaged with a female thread provided in the plug hole of the internal-combustion engine, and then a spark plug main body is pressed and tightened (i.e., secured) in the internal-combustion engine. For example, see Japanese Patent Application Laid-Open (kokai) No.2002-050449.
A spark plug having a composition wherein a spark plug main body is fixed to an internal-combustion engine using the above-described cylindrical pressing member has the following problems. That is, such a spark plug does not have a male thread on the metal shell for fixing the spark plug to the internal-combustion engine. Therefore, a spark plug main body must be pulled out from a plug hole of the internal-combustion engine, after removing the cylindrical pressing member, when removing the spark plug from the internal-combustion engine. However, the spark plug main body tends to get stuck in the plug hole due to soot and sediment piled up in a clearance between the plug hole and the metal shell, and thus it is difficult to remove the spark plug main body from the plug hole.
The present invention is accomplished in light of the above-mentioned problems, and an object of the present invention is to provide a spark plug that can be easily removed from a plug hole of the internal-combustion engine.